Today there is an increasing global desire to reduce greenhouse gas emissions and develop clean alternative vehicle fuels. Methane (CH4), the primary component of natural gas (NG), is of particular interest as it is abundant and has lower carbon dioxide (CO2) emission and more efficient combustion than other hydrocarbons due its high H/C ratio. Biogases, including landfill gas, are also seen as promising renewable energy resources, but, like NG, they contain significant amounts of water, CO2, and hydrogen sulfide (H2S) which must be removed before being transported, stored, and burned as a fuel. For example, NG must contain less than 1-2% CO2 and 4 ppm H2S to meet fuel gas specifications for pipeline transportation. Within many industries, gas dehydration and removal of CO2 and H2S remain some of the most intensive and challenging separations, in part due to the intolerance of many technologies to water.
Available technologies for refining NG and other biogases are often costly, multi-stage processes. Amine scrubbing is a common liquid phase system used to remove acid gases such as CO2 and H2S from NG. However, stagnant historical operating efficiencies, and the excessive oxidative degradation, evaporation, and the corrosive nature of the alkanolamine aqueous solutions create a myriad of performance, safety, and environmental concerns. Solid, porous material systems, such as zeolite and metal organic frameworks (MOFs), offer more environmentally friendly alternatives for CO2 capture, but require cumbersome, multi-stage processes. For example, zeolite has single-species selectivity for CO2 and cyclic adsorption performance in the presence of moisture that require prior dehydration and H2S removal stages. MOFs, similarly, can be designed for CO2 capture but exhibit prohibitively low selectivity for acid gases such as H2S. Further, MOFs exhibit low selectivity and capture at low CO2 partial pressures.
MOFs generally include porous crystals which are assembled from modular molecular building blocks, and provide a wide array of advantageous material properties, including high surface area, porosity, stability, and sorption potential. While the available building block options, and combinations thereof, are virtually limitless, such potential highlights the statistical difficulty in identifying and assembling MOFs with desired and particularized material properties and multi-faceted functionality. For example, many MOFs exhibit high selectivity towards a particular molecular species, but are highly intolerant to water and H2S.